Operation of a key button on the keyboard causes localized deflection of one of two membranes to bring the appropriate contact pads into contact with one another. One problem with such an arrangement is that of contact bounce and acoustic noise. The article at page 2773 of the IBM Technical Disclosure Bulletin Vol. 28, No. 7 (Dec. 1985) illustrates how the acoustic noise and contact bounce characteristics of such a membrane keyboard can be improved by providing a neoprene membrane serving to dampen oscillations of the activated key. Such an arrangement significantly increases the "quality" of the keyboard but does, of course, add to the expense of the keyboard. It has been found that the acoustic noise and contact bounce characteristics can be at least maintained with a significant decrease in the cost of the keyboard by replacing the neoprene with expanding ink pads printed onto the upper surface of the membrane at least adjacent the activating keys.
Expanding inks have been employed mainly in the past for decorative purposes on articles of clothing such as T-shirts and sports shoes. Offenlegungsschrift DE 324666 - A1 describes an electrical membrane switch in which the two membranes are separated by a spacing layer consisting of a layer of ink made from insulating material and printed onto one of the membranes and bonded to the other, either by means of an adhesive or by means of heat and pressure, to produce a laminated assembly. The non-expanding ink is formed from thermoplastic resin in powder form dispersed in a liquid plasticiser with a stabilizing agent, viscosity control agent and a pigment. The spacing layer serves no purpose other than to bond the two membranes together but in spaced relationship.